1. Field of the Invention
The present invention relates to an apparatus for producing plasma of high density, and to apparatus that uses the same. For example, the plasma producing apparatus is employed in a doping apparatus, etching apparatus, sputtering apparatus, and film forming apparatus.
2. Description of the Related Art
In recent years, manufacture of a transistor from a thin film (thin film transistor: TFT) has been widely researched. In manufacture of a TFT, film formation, etching, and ion introduction are some of indispensable processes and plasma is often utilized in these steps. To be specific, highly reactive ions or radicals are utilized in etching and film formation, particles having large kinetic energy are utilized in sputtering, and charged particles in plasma are utilized in etching and ion implantation. A TFT is manufactured through these steps using sputtering, plasma CVD, etching, ion doping, and the like.
There are several methods to produce plasma. One is to apply a uniform electric field between two planar electrodes that are set in parallel to each other (FIG. 4A, the electrode denoted by 51 is a cathode electrode and one denoted by 52 is an anode electrode). Another method is to use a directly-heated thermionic cathode electrode 53 and heat a high-melting point metal such as tungsten up to about 2500° C. to make the metal emit thermoelectrons (FIG. 4B). Still another method is indirect heating from the back using a substance having a small work function for a surface of a cathode electrode (FIG. 4C). Yet still another method is to produce plasma of high density using as a cathode electrode a cylindrical hollow electrode that is open at one end open and closed on the other end (hollow cathode discharge 54)(FIG. 4D). FIG. 4E shows a method of producing plasma by placing many permanent magnets along a wall of a plasma chamber and utilizing magnetic fields present on some parts of the wall surface (surface magnetic fields or multipolar magnetic fields). FIG. 4F shows a plasma producing method called magnetron discharge in which a magnetic field is applied in parallel to a planar cathode electrode face for electric discharge. These are examples of using direct current discharge but plasma can be produced also by alternating current discharge.
Major examples of producing plasma by alternating current are shown in FIGS. 5A to 5F. FIG. 5A shows a method called capacitively coupled plasma in which high frequency wave or microwave is applied to parallel planar electrodes. FIGS. 5B and 5C each show a method called inductively coupled plasma in which a high frequency current is let flow in a helical coil or a spiral coil. FIG. 5D shows a method called surface wave plasma in which surface wave is excited by irradiating electromagnetic wave onto high density plasma and is brought against intense microwave. FIG. 5E shows a method called ECR (electron cyclotron resonance) plasma which utilizes electron cyclotron resonance in a magnetic field. FIG. 5F shows a method called helicon wave plasma in which a high frequency current whose frequency is sufficiently lower than the electron cyclotron frequency is let flow in an antenna.
An ion doping apparatus is used mainly in manufacture of a thin film transistor (TFT) on a large-area glass substrate. In doping for forming a source region or a drain region, the dose necessary is on the order of 1015 ions/cm2. In channel doping and doping for forming an LDD (lightly doped drain) region, the dose necessary is 1×1012 to 1×1014 ions/cm2. Since the necessary dose in one type of regions is different from the dose needed in the other type of regions by three orders of magnitude, an ability to control high current density as well as low current density is required for an ion doping apparatus. For instance, an ion doping apparatus with a DC ion source using a filament is popular because the apparatus can handle a wide range of current density.
A problem of the ion doping apparatus with a DC ion source using a filament is that a filament has a short lifespan. Replacing a filament lowers the availability of the apparatus and reduces the throughput. In addition, it is desirable to replace a filament as less frequently as possible because many ion doping apparatuses use phosphine, diborane, and other harmful gas for material gas and it can contaminate a clean room and adversly affect human health. However, the lifespan of a filament is even shorter in the high dose treatment for forming a source region or a drain region because the ion coulomb density is increased in this treatment in order to raise the throughput. In addition, a filament that is long in use is gradually degraded and a film is adhered to the filament surface. These changes in a filament cause a change with time in field emission characteristic from the filament surface and a change in ion species ratio in plasma, thereby making stable dose control impossible. Stabilizing plasma is the most important objective not only in an ion doping apparatus but in every apparatus that deals with plasma.